KEVOS® MAKEOS · Materials
Differential Thermal Expansion
How does a fit clearance change when the bore and shaft materials expand differently?
Calculate how the diametral clearance between a bore and a shaft of different materials changes when both change temperature by the same amount.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal fit diameter | mm | Nominal diameter of the fit. |
| Bore (housing) expansion coefficient | ×10⁻⁶ /°C | Linear expansion coefficient of the housing material over the temperature range, for example about 23 × 10⁻⁶ /°C for many aluminium alloys. |
| Shaft expansion coefficient | ×10⁻⁶ /°C | Linear expansion coefficient of the shaft material, for example about 12 × 10⁻⁶ /°C for many carbon steels. |
| Uniform temperature change from assembly | °C | Temperature change of both parts from the assembly temperature; negative for cooling. |
Method and formulas
- Growth of each part = diameter × coefficient × 10⁻⁶ × temperature change.
- Change in clearance = bore growth − shaft growth; a positive value loosens the fit.
- Example: 50 mm, 23 and 12 × 10⁻⁶ /°C and +60 °C give 0.069 − 0.036 = +0.033 mm more clearance.
Limitations
- Both parts are assumed to be at one uniform temperature. In service they often differ, which can dominate the result.
- Coefficients are constant over the range; real values change with temperature and alloy. Stresses in an interference fit are not calculated.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a fit loosens or tightens at operating temperature
- Compare housing materials
- Estimate a temperature for assembly
Who it is for
- Design engineers
- Maintenance engineers
What you get
- Change in diametral clearance (mm)
- Bore and shaft growth (mm)
Interpreting the result
- Add the result to the assembly clearance to estimate the running clearance; a negative running clearance means interference.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 375–379. Topic reference; no source tables or recommended settings are reproduced.
- NPL, Fundamental Good Practice in Dimensional Metrology, GPG 80 — https://eprintspublications.npl.co.uk/3360/1/mgpg80.pdf
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